Fibrodysplasia ossificans progressiva (FOP) is a heritable disorder of mesenchymal cell differentiation in which heterotopic bone develops in the connective tissue surrounding skeletal muscle. In our studies on the basic cellular defect we have discovered that the cellular transformation characteristic of FOP differs from the non-genetic, non-progressive disorder, myositis ossificans traumatica. Clinical studies have demonstrated a previously undescribed primary or secondary amenorrhea in affected females, and the pathophysiology of the associated condition is currently under study. A limited clinial trial is underway to determine the efficacy of a synthetic retinoid, a drug suggested by in vitro studies to be a potentially useful agent in the treatment of FOP.